GEOLOGIC MAP OF THE DIETRICH, DIETRICH BUTTE, AND OWINZA 7.5’ QUADRANGLES, TWIN FALLS URBAN AREA, IDAHO: EDMAP

The Dietrich, Dietrich Butte, and Owinza 7.5’ quadrangles are situated just east of Shoshone, Idaho, in the Twin Falls Urban Area. This area has a rapidly growing population that presents an increasing demand on water resources, especially the Snake River Plain aquifer. Previous work suggests that vertical movement of groundwater occurs along joints and faults within the lava flows, while horizontal movement occurs along porous interflow zones between basalt flows.

Our detailed mapping in the central SRP reveals new details of groundwater recharge and flow that have not been identified in previous studies, and allow us to formulate a new model for the formation of coarse alluvial aquifers with high conductivity in the central SRP. Groundwater flow in the SRP is controlled by relatively porous horizons between lava flows. Our mapping shows that thick alluvial horizons are most common where adjacent volcanic edifices abut one another and overlap. This overlap creates a moat which controls the location of surface stream drainages, which fill with coarse alluvium. Younger lava flows are channeled into these drainages, displacing the streams and covering the alluvium with relatively thick, semi-permeable caps. This forms elongate aquifers with extremely high conductivities that follow the pre-existing paleo-drainage. Reconstruction of basaltic volcanism through time using detailed geologic maps allows us to predict the location of paleo-drainage and elongate alluvial aquifers. This ability to predict aquifer location will prove to be a valuable tool as increased demands are placed upon the ground water supply by agriculture and population growth. In addition, our mapping reveals that young basalt flows seldom exhibit channelized flow that connects with major through-going streams. Instead, the rugged volcanic topography of ridges, flow fronts, lava channels, and collapse pits trap precipitation, which must either evaporate or percolate into the fractured lavas to recharge the local groundwater. These young volcanic features constitute “negative basins” of interior drainage, despite their topographic emergence. We suggest that these young basalts represent recharge zones that can be easily mapped and distinguished from older flows that display well developed external drainage.